1. Field of the Invention
Broadly speaking, this invention relates to energy conversion. More particularly, this invention relates to methods and apparatus for converting rotational kinetic energy into thermal energy.
2. Description of the Prior Art
The prior art approach most often utilized for the conversion of the kinetic energy contained in a moving stream of fluid involves the use of turbines, windmills and the like. Such prior art apparatus will hereinafter be referred to generically as a windmill, whether or not air is the ambient flowing fluid used.
Prior art windmills typically comprise a rotor which is rotated by the impinging wind and a shaft secured to the rotor to couple the rotational energy of the rotor to some suitable output device. As is well known, such rotational energy may be used, for example, to drive a generator for the production of electricity, to pump water for irrigation purposes, etc., etc.,
Prior art windmills are generally either horizontal or vertical windmills having rotors for rotation about horizontal and vertical axes, respectively. More specifically, horizontal windmills have their axes oriented parallel to the direction of the wind while vertical windmills have their axes oriented normal to the wind. This difference in orientation produces some characteristic differences between horizontal and vertical windmills which should be discussed.
First, to achieve maximum efficiency, horizontal windmills must be provided with some suitable means for continually maintaining the rotor axis parallel to the direction of the wind. Vertical windmills, on the other hand, are omni-directional and, thus, do not need to be continually repositioned.
Furthermore, horizontal windmills are provided with rotor blades that rotate in a perpendicular plane. At some wind velocities the speed of the windmill rotation is considerably greater than the velocity of the wind. In order to keep destructive centrifugal forces to a minimum at such high wind velocities, horizontal windmill rotor blades are usually "feathered," with a resultant decrease in their power output. It is thus necessary to sacrifice power output in order to maintain the integrity of a horizontal windmill at high wind velocities. Vertical windmills, on the other hand, do not usually need to be feathered to prevent destruction since they do not rotate at speeds which are greater than the velocity of the wind. Concern for the integrity of prior art vertical windmills generally arises only at extremely high wind velocities, velocities which are well beyond those magnitudes necessitating feathering of horizontal windmills.
Furthermore, prior art horizontal windmills have an inherently high inertia and do not generally begin to rotate and generate power until the wind velocity is at a relatively high magnitude. Vertical windmills, on the other hand, tend to have a lower inertia and are, thus, capable of beginning to rotate and generating power at lower wind velocities.
In view of the above, it is generally recognized by those skilled in the art that vertical windmills possess characteristics which make them considerably more attractive than horizontal windmills for efficient conversion of the kinetic energy contained in the wind. Furthermore, it is known that the power output of a vertical windmill is, in part, a function of its rotational speed. It is also known that this relationship is not linear but, rather, the energy output will be at a maximum at a certain predetermined rotational speed, that speed being a function of the design of the particular vertical windmill. For example, a Darrieus-type windmill rotor has a maximum power output when the ratio of the blade tip speed to the free-flow wind velocity is approximately six, while a Savonius-type rotor has a maximum energy power output at a ratio of approximately 1.
While it is generally recognized then that the energy output of a vertical windmill is maximized at a certain relative rotational speed, there is no known prior art device which will maintain this power output at a maximum level.